This invention relates to semiconductor light emitting devices and manufacturing methods therefor, and more particularly to packaging and packaging methods for semiconductor light emitting devices.
Semiconductor light emitting devices, such as Light Emitting Diodes (LEDs) or laser diodes, are widely used for many applications. As is well known to those having skill in the art, a semiconductor light emitting device includes one or more semiconductor layers that are configured to emit coherent and/or incoherent light upon energization thereof. It is also known that the semiconductor light emitting device generally is packaged to provide external electrical connections, heat sinking, lenses or waveguides, environmental protection and/or other functions.
For example, it is known to provide a two-piece package for a semiconductor light emitting device, wherein the semiconductor light emitting device is mounted on a substrate that comprises alumina, aluminum nitride and/or other materials, which include electrical traces thereon, to provide external connections for the semiconductor light emitting device. A second substrate, which may comprise silver plated copper, is mounted on the first substrate, for example, using glue, surrounding the semiconductor light emitting device. A lens may be placed on the second substrate over the semiconductor light emitting device. Light emitting diodes with two-piece packages as described above are described in Application Serial No. US 2004/0041222 A1 to Loh, entitled Power Surface Mount Light Emitting Die Package, published Mar. 4, 2004, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
With multipart mounting packages for semiconductor light emitting devices, the different parts are typically made of different materials. As a result, the thermal impedance for such packages may be higher and a thermal mismatch may result among the various components within a package that may cause reliability problems with a package. For example, problems may result at an interface between a copper metal of a heat sink or cavity with a plastic of a body in which such a heat sink or cavity is mounted. In addition, assembly may be more complicated because of increased piece part counts for the package. In addition, where a sheet metal optical cavity is utilized, a cavity typically can only be fabricated in a limited range of depth and shape configurations. Such multipart packages may also have a larger optical cavity space, resulting in greater volumes of encapsulant being used, which may increase problems related to delamination and/or formation of bubbles within the encapsulant during temperature cycles.
The use of a pre-molded lens attached by adhesive may encounter some problems in robustness and reliability of the finished product. For example, the manufacturing process for such devices may be inherently inconsistent and the resultant package may be less robust and/or reliable. It is also known to form the lens using a dispensing method capitalizing on the viscosity of a resin used in forming the lens.
In some applications, it may be preferred to mount the LED on a surface of a substrate, such as a ceramic substrate, a metal core printed circuit board (MCPCB), a flexible circuit substrate and/or a lead frame, without use of a reflector cup. However, where no such structure is provided, it may be more difficult to form and/or secure a lens as various of the approaches described above may not be well suited to use where the LED is not positioned within a cavity.
It is also known to use transfer molding of epoxy to encapsulate certain low power LED packages, such as miniature surface mountable devices available from from Hewlett Packard Corporation. The epoxy on such devices may provide the structural strength to the package as well as encapsulating the devices inside. However, epoxy tends to be degraded by the electromagnetic energy of blue light, generally generated by some semiconductor light emitting devices, and may become less transmissive to light as a result. The resulting package may, therefore, become dimmer over a relatively short period of time. As such, epoxy may be a less attractive option for encapsulating devices that emit blue light. In addition, epoxy generally has a Coefficient of Thermal Expansion (CTE) mismatch problem with silicone soft gel, which may be used to junction coat the LED chips and their bond wires as the first layer of encapsulant.
It is also known to use casting to encapsulate LED devices with epoxy. This process typically can only be applied to an open chamber, where curing may occur with the epoxy contained in a cup and a lead frame may be inserted inside the cup and be casted when the epoxy is cured. During curing, a level of liquid epoxy is generally free to adjust itself as a result of chemical reactions and shrinkage in volume.
Another approach uses compression molded lenses formed of silicone. Using compression molding, an array of compression molded lenses may be placed over a matching array of LED chips on a substrate or wafer. However, conventional compression molding of lenses generally requires the use of electrical contacts on the back side, rather than the front side, of the substrate as the molding material may extend across and limit formation of electrical connections with front side contacts.
Packaging of semiconductor light emitting devices may add cost to the resulting packaged device due to the precision required for various operations. The costs typically increase as packaged light emitting devices having different optical properties are required. While compression molding technology has been proposed that could lower the cost for forming packaged light emitting devices, benefits of this technology have not been fully realized. For example, such techniques have generally only been used to produce simple lenses made of one material.